Electrophoretic Display and a Method and Apparatus for Driving an Electrophoretic Display

ABSTRACT

An electrophoretic display, in which a driving method is employed whereby a sequence of discrete picture potential differences in the form of a driving waveform is supplied for enabling the charged particles ( 6 ) of the display to occupy a position for displaying an image, the position being one of a number of positions between the electrodes ( 3, 4 ). The driving waveform consists of a sequence of image update signals including a picture potential difference, the image update signals being separated by dwell times, and the method includes the step of generating one or more shaking pulses during the dwell times. Such shaking pulses may be generated immediately after each image update signal or they may comprise regular shaking pulses generated at predetermined intervals along the waveform.

This invention relates to an electrophoretic display comprising anelectrophoretic material comprising charged particles in a fluid, aplurality of picture elements, first and second electrodes associatedwith each picture element for receiving a potential difference, thecharged particles being able to occupy a position being one of aplurality of positions between the electrodes, and drive means arrangedto supply a sequence of picture potential differences in the form of adriving waveform for enabling the charged particles to occupy one of thepositions for displaying an image.

An electrophoretic display comprises an electrophoretic mediumconsisting of charged particles in a fluid, a plurality of pictureelements (pixels) arranged in a matrix, first and second electrodesassociated with each pixel, and a voltage driver for applying apotential difference to the electrodes of each pixel to cause it tooccupy a position between the electrodes, depending on the value andduration of the applied potential difference, so as to display apicture.

In more detail, an electrophoretic display device is a matrix displaywith a matrix of pixels which area associated with intersections ofcrossing data electrodes and select electrodes. A grey level, or levelof colourisation of a pixel, depends on the time a drive voltage of aparticular level is present across the pixel. Dependent on the polarityof the drive voltage, the optical state of the pixel changes from itspresent optical state continuously towards one of the two limitsituations, e.g. one type of all charged particles is near the top ornear the bottom of the pixel. Grey scales are obtained by controllingthe time the voltage is present across the pixel.

Usually, all of the pixels are selected line by line by supplyingappropriate voltages to the select electrodes. The data is supplied inparallel via the data electrodes to the pixels associated with theselected line. If the display is an active matrix display, the selectelectrodes will activate active elements such as TFT's, MIM's, diodes,which in turn allow data to be supplied to the pixel. The time requiredto select all the pixels of the matrix display once is called thesub-frame period. A particular pixel either receives a positive drivevoltage, a negative drive voltage, or a zero drive voltage during thewhole sub-frame period, dependent on the change in optical staterequired to be effected. A zero drive voltage should be applied to apixel if no change in optical state is required to be effected.

FIGS. 10 and 11 illustrate an exemplary embodiment of a display panel 1having a first substrate 8, a second opposed substrate 9, and aplurality of picture elements 2. In one embodiment, the picture elements2 might be arranged along substantially straight lines in atwo-dimensional structure. In another embodiment, the picture elements 2might be arranged in a honeycomb arrangement.

An electrophoretic medium 5, having charged particles 6 in a fluid, ispresent between the substrates 8, 9. A first and second electrode 3, 4are associated with each picture element 2 for receiving a potentialdifference. In the arrangement illustrated in FIG. 11, the firstsubstrate 8 has for each picture element 2 a first electrode 3, and thesecond substrate 9 has for each picture element 2 a second electrode 4.The charged particles 6 are able to occupy extreme positions near theelectrodes 3, 4, and intermediate positions between the electrodes 3, 4.Each picture element 2 has an appearance determined by the position ofthe charged particles 6 between the electrodes 3, 4.

Electrophoretic media are known per se from, for example, U.S. Pat. No.5,961,804, U.S. Pat. No. 6,120,839 and U.S. Pat. No. 6,130,774, and canbe obtained from, for example, E Ink Corporation. As an example, theelectrophoretic medium 5 might comprise negatively charged blackparticles 6 in a white fluid. When the charged particles 6 are in afirst extreme position, i.e. near the first electrode 3, as a result ofpotential difference applied to the electrodes 3, 4 of, for example, 15Volts, the appearance of the picture element 2 is for example, white inthe case that the picture element 2 is observed from the side of thesecond substrate 9.

When the charged particles 6 are in a second extreme position, i.e. nearthe second electrode 4, as a result of a potential difference applied tothe electrodes 3, 4 of, for example, −15 Volts, the appearance of thepicture element is black. When the charged particles 6 are in one of theintermediate positions, i.e. between the electrodes 3, 4, the pictureelement 2 has one of a plurality of intermediate appearances, forexample, light grey, mid-grey and dark grey, which are grey levelsbetween black and white.

FIG. 12 illustrates part of a typical conventional random greyscaletransition sequence using a pulse width modulated transition matrix.Between the image state n and the image state n+1, there is always acertain time period (dwell time) available which may be anything from afew seconds to a few minutes, dependent on different users.

In general, in order to generate grey scales (or intermediate colourstates), a frame period is defined comprising a plurality of sub-frames,and the grey scales of an image can be reproduced by selecting per pixelduring how many sub-frames the pixel should receive which drive voltage(positive, zero, or negative). Usually, the sub-frames are all of thesame duration, but they can be selected to vary, if desired. In otherwords, typically grey scales are generated by using a fixed value drivevoltage (positive, negative, or zero) and a variable duration of driveperiods. Alternatively, variable drive voltages magnitudes could beapplied to generate grey levels.

In a display using electrophoretic foil, many insulating layers arepresent between the ITO-electrodes, which layers become charged as aresult of the potential differences. The charge present at theinsulating layers is determined by the charge initially present at theinsulating layers and the subsequent history of the potentialdifferences. Therefore, the positions of the particles depend not onlyon the potential differences being applied, but also on the history ofthe potential differences. As a result, significant image retention canoccur, and the pictures subsequently being displayed according to imagedata differ significantly from the pictures which represent an exactrepresentation of the image data.

As stated above, grey levels in electrophoretic displays are generallycreated by applying voltage pulses for specified time periods. They arestrongly influenced by image history, dwell time, temperature, humidity,lateral inhomogeneity of the electrophoretic foils, etc. In order toconsider the complete history, driving schemes based on the transitionmatrix have been proposed. In such an arrangement, a matrix look-uptable (LUT) is required, in which driving signals for a greyscaletransition with different image history are predetermined. However,build up of remnant dc voltages after a pixel is driven from one greylevel to another is unavoidable because the choice of the drivingvoltage level is generally based on the requirement for the grey value.The remnant dc voltages, especially after integration after multiplegreyscale transitions, may result in severe image retention and shortenthe life of the display.

It is therefore an object of the present invention to provide a methodand apparatus which overcomes the problems outlined above, to reduceimage retention in an electrophoretic display.

In accordance with the present invention, there is provided a displayapparatus, comprising:

-   an electrophoretic material comprising charged particles in a fluid;    -   a plurality of picture elements;    -   first and second electrodes associated with each picture element        for receiving a potential difference, said charged particles        being able to occupy a position being one of a plurality of        positions between said electrodes; and    -   drive means arranged to supply a sequence of picture potential        differences in the form of a driving waveform for enabling said        charged particles to occupy one of said positions for displaying        an image, the driving waveform consisting of a sequence of image        update signals including a picture potential difference, the        image update signals being separated by dwell times, wherein one        or more shaking pulses are generated during the dwell times.

Also in accordance with the present invention, there is provided amethod of driving a display apparatus, the apparatus comprising:

-   -   an electrophoretic material comprising charged particles in a        fluid;    -   a plurality of picture elements;    -   first and second electrodes associated with each picture element        for receiving a potential difference, said charged particles        being able to occupy a position being one of a plurality of        positions between said electrodes; and    -   drive means arranged to supply a sequence of picture potential        differences in the form of a driving waveform for enabling said        charged particles to occupy one of said positions for displaying        an image, the driving waveform consisting of a sequence of image        update signals including a picture potential difference, the        image update signals being separated by dwell times; the method        including the step of generating one or more shaking pulses        during the dwell times.

Still further in accordance with the present invention, there isprovided driving apparatus for driving a display apparatus, the displayapparatus comprising:

-   -   an electrophoretic material comprising charged particles in a        fluid;    -   a plurality of picture elements; and    -   first and second electrodes associated with each picture element        for receiving a potential difference, said charged particles        being able to occupy a position being one of a plurality of        positions between said electrodes;        wherein the driving apparatus is arranged to supply a sequence        of picture potential differences in the form of a driving        waveform for enabling said charged particles to occupy one of        said positions for displaying an image, the driving waveform        consisting of a sequence of image update signals including a        picture potential difference, the image update signals being        separated by dwell times, the driving apparatus further        comprising means for generating one or more shaking pulses        during the dwell times.

In one aspect, the one or more shaking pulses may be generated,preferably substantially immediately, following each image updatesignal.

Each image update signal preferably consists of a reset pulse and agreyscale driving pulse. One or more shaking pulses may also begenerated as part of the image update signal, for example, between thereset pulse and the greyscale driving pulse and/or substantiallyimmediately prior to the reset pulse, as part of the image sequence.

In one preferred embodiment of the invention, a sequence of shakingpulses may be generated following each image update signal, the energyof the shaking pulses, defined as the product of (voltagemagnitude)×(time), of each sequence decreasing progressively during thesequence, such that the energy of the first few pulses of the sequenceis greater than that of the final few pulses of the same sequence.

In accordance with a second aspect of the invention, the one or moreshaking pulses may comprise regular shaking pulses, which may begenerated at predetermined, preferably substantially equi-distant,intervals along the driving waveform.

Each image update signal may also be immediately preceded by one or moreshaking pulses. Means may be provided to temporarily stop generation ofthe one or more regular shaking pulses during an image update sequence.

Charge recycling means may be provided so as to reduce powerconsumption. Alternatively, or in addition, the apparatus may bearranged to operate in one of at least two modes, a first mode in whichgeneration of the regular shaking pulses is enabled and a second mode inwhich generation of the regular shaking pulses is disabled, such thatpower consumption is reduced in the second mode relative to that in thefirst.

The term “shaking pulses” is used herein to refer to as one shortvoltage pulse or a series of short, alternating negative and positive,voltage pulses. A shaking pulse is a single polarity voltage pulserepresenting an energy value sufficient to release particles at one ofthe two extreme positions but insufficient to move the particles fromone of the extreme positions to the other extreme position between thetwo electrodes. When a single shaking pulse is used, its polarity ispreferably opposite to the first pulse of the subsequent drive waveform.

These and other aspects of the invention will be apparent from, andelucidated with reference to, the embodiments described hereinafter.

Embodiments of the present invention will now be described by way ofexamples only and with reference to the accompanying drawings, in which:

FIG. 1 illustrates schematically a cyclic rail-stabilized driving methodfor an electrophoretic display having four optical states: white (W),light grey (G2), dark grey (G1) and black (B);

FIG. 2 a illustrates schematically a driving waveform generated by aknown method;

FIG. 2 b illustrates schematically a driving waveform generated by amethod according to a first exemplary embodiment of the presentinvention;

FIG. 3 illustrates schematically a driving waveform generated by amethod according to a second exemplary embodiment of the presentinvention.

FIG. 4 illustrates schematically a driving waveform generated by amethod according to a third exemplary embodiment of the presentinvention, in comparison with a driving waveform generated by a knownmethod.

FIG. 5 illustrates schematically a driving waveform generated by amethod according to a fourth exemplary embodiment of the presentinvention;

FIG. 6 illustrates schematically a driving waveform generated by amethod according to a fifth exemplary embodiment of the presentinvention;

FIG. 7 illustrates schematically a driving waveform generated by a knownmethod;.

FIG. 8 illustrates schematically a driving waveform generated by amethod according to a sixth exemplary embodiment of the presentinvention;

FIG. 9 illustrates schematically a driving waveform generated by amethod according to a seventh exemplary embodiment of the presentinvention;

FIG. 10 is a schematic front view of a display panel according to anexemplary embodiment of the present invention;

FIG. 11 is a schematic cross-sectional view along II-II of FIG. 10; and

FIG. 12 illustrates part of a typical greyscale transition sequenceusing a voltage modulated transition matrix according to the prior art.

Thus, as explained in detail above, grey levels in an electrophoreticdisplay are generally created by applying voltage pulses to theelectrodes of the respective picture elements for specified timeperiods. The accuracy of the greyscales in electrophoretic displays isstrongly influenced by image history, dwell time, temperature, humidity,lateral inhomogeneity of the electrophoretic foils, etc.

It has been demonstrated that accurate grey levels can be achieved usinga so-called rail-stabilized approach. This means that the grey levelsare always achieved via one of the two extreme optical states (say blackor white) or “rails”, irrespective of the image sequence itself.

In order to achieve substantially dc-balanced driving, a cyclicrail-stabilized greyscale concept has recently been proposed, and it isillustrated schematically in FIG. 1 of the drawings. In this method, asstated above, the “ink” must always follow the same optical path betweenthe two extreme optical states, say full black or full white (i.e. thetwo rails), regardless of the image sequence, as indicated by the arrowsin FIG. 1. In the illustrated example, the display has four differentstates: black (B), dark grey (G1), light grey (G2) and white (W).

A driving method using a single over-reset voltage pulse has recentlybeen proposed for driving an electrophoretic display, and is shownschematically in FIG. 2 a for image transitions to dark grey from black(B), dark grey (G1), light grey (G2) and white (W). The pulse sequenceusually consists of four portions: a first sequence of shaking pulses, areset pulse, a second sequence of shaking pulses, and a greyscaledriving pulse, whereby the second sequence of driving pulses occursbetween the reset and greyscale driving pulses.

The reset pulse is longer than the minimum time required for switchingthe “ink” from full black or white to the opposite rail state, therebyensuring that the previous image is fully erased during a new imageupdate. Regardless of the image update sequence, both the first andsecond sequences of shaking pulses are required to reduce dwell time andimage history effects, thereby reducing the image retention andincreasing greyscale accuracy.

However, image retention may still be unacceptably visible if the imageupdate time is limited to less than, say, 1 second and, although suchimage retention can be reduced by the provision of a longer reset pulseand/or more shaking pulses, this would obviously increase the imageupdate time beyond the required level.

Thus, in accordance with a first aspect of the invention, a drivingmethod is proposed an electrophoretic display having at least fourgreyscale levels (hereinafter referred to as “two bits greyscale”) inwhich shaking pulses are provided substantially immediately after eachgreyscale driving pulse. Thus, in the preferred method, the drivingpulse sequence will still consist of four portions: a first sequence ofshaking pulses, a reset pulse, a second sequence of shaking pulses(between the reset and greyscale driving pulses) and a greyscale drivingpulse, as described with reference to FIG. 2 a, but with the addition ofa third sequence of shaking pulses during the dwell time immediatelyfollowing the greyscale driving pulse. It will be apparent to a personskilled in the art that the energy involved in the third sequence ofshaking pulses should be sufficient to move the particles a relativelysmall distance but insufficient to move the particles over anysignificant distance such that visible optical flicker is avoided.

The third sequence of shaking pulses are beneficially applied to thewhole display at the same time by means of, for example, hardwareshaking, where pixels are provided with voltage pulses independent ofthe image update sequence. In this way, image retention can be reducedwithout increasing the total image update time.

In more detail, and referring to FIG. 2 b of the drawings, in anexemplary embodiment of the invention, an electrophoretic display hastwo rail states and at least two bits grey level, i.e. black (B), darkgrey (G1), light grey (G2) and white (W). Four transitions to G1 statefrom W, G2, G1 and B are realised using two types of pulse sequenceswhen the over-reset technique described above is used for resetting thedisplay, with a long sequence being required for the transition from G2to W or G1, and a shorter sequence being used for transitions from G1 orB to G1.

In the illustrated example, for all types of image transition, eachsequence consists of five portions, the image update sequencecomprising, as before, a first sequence of shaking pulses, a resetpulse, a second sequence of shaking pulses (between the reset andgreyscale driving pulses), and a greyscale driving pulse, and a fifthportion, comprising a third sequence of shaking pulses which aregenerated after the completion of an image update, i.e. during the dwelltime immediately following an image update. Thus, because image updatetime is influenced only by the first four portions of the sequencedescribed above, it is not adversely affected by the addition of thethird sequence of shaking pulses, as the effect of the shaking pulseshould be invisible to the user. Thus, in summary, the embodimentdescribed with reference to FIG. 2 b of the drawings, results in areduced image retention without increasing image update time (as thefinal set of shaking pulses are not very visible to a viewer).

It is important to limit the visibility of optical flickers which may becaused by the third sequence of shaking pulses, by properly controllingthe pulse time or amplitude of a shaking pulse so that the energyinvolved is sufficient to move the particles a relatively small distancebut insufficient to move the particles any significant distance.

In accordance with a second exemplary embodiment of the presentinvention, as illustrated schematically in FIG. 3, a third sequence ofshaking pulses is generated immediately after an image update sequence,as in the exemplary embodiment described with reference to FIG. 2 b, butin this case, this third sequence of shaking pulses has a variableamplitude or pulse length time, such that in this case, the energyinvolved in the initial pulses in a sequence is greater than thatinvolved in the final pulses of the sequence. Thus, the exemplaryembodiment of the invention described with reference to FIG. 3 of thedrawings results in a reduced image retention without an increase inimage update time (as the visibility of the final shaking pulse is stillfurther reduced relative to that of the drive waveform illustrated inFIG. 2 b, due to its decreasing energy).

In accordance with a third exemplary embodiment of the presentinvention, as illustrated schematically in FIG. 4 of the drawings(right-hand side), the length of the reset pulse used in each imageupdate sequence may be variable and proportional to the distance overwhich the ink is required to move in the vertical direction in order toeffect an image transition. By way of clarification, the comparabledriving waveforms generated by a known driving method are illustrated inthe left-hand drawing of FIG. 4.

As an example, consider the situation where, if the image update data ispulse width modulated (PWM), a full pulse width (FPW) is required toeffect a transition from white to black, but only ⅔ FPW is required toeffect a transition from G2 to black, and only ⅓ FPW is required to gofrom G1 to black. Thus, a full reset pulse is used in the image updatesequence for the white to black transition, ⅔ of that pulse length isused in the image update sequence for the G2 to black transition, ⅓ ofthat pulse length is used in the image update sequence for the G1 toblack transition, and no reset pulse is used for the black to G1transition, i.e. no “over-reset” technique is used. These waveforms areusable when, for example, transition matrix-based methods are used, inwhich previous images are considered in the determination of the energyimpulses (time×voltage) of pulses required for the next image. Inaddition, these waveforms are usable when the electrophoretic materialsused in the display are insensitive to the image history and/or dwelltime.

As shown, a third sequence of shaking pulses is added to the waveformduring the dwell time immediately following the greyscale driving pulse(or complete image update sequence). As before, because image updatetime is influenced only by the image update sequence as described abovewith reference to the first exemplary embodiment of the invention, it isnot adversely affected by the addition of the third sequence of shakingpulses during the dwell time immediately following the image updatesequence.

Once again, it is important to limit the visibility of optical flickerswhich may be caused by the third sequence of shaking pulses, by properlycontrolling the pulse time or amplitude of a shaking pulse so that theenergy involved is sufficient to move the particles a relatively smalldistance but insufficient to move the particles any significantdistance. As before, the third sequence of shaking pulses may bebeneficially applied to the whole display at the same time by means of,for example, hardware shaking, regardless of the image update sequence.In this way, image retention can be reduced without increasing the totalimage update time.

Referring to FIG. 5 of the drawings, a driving waveform generated by afourth exemplary embodiment of the present invention is similar in manyrespects to that described with reference to, and illustratedschematically by, FIG. 4 of the drawings. However, in this case, adifferent type of shaking pulse is used as the third sequence of shakingpulses, whereby the amplitude or pulse length time decreases over thesequence, i.e. the energy involved in the initial pulses of the sequenceis greater than that of the final pulses of the sequence, as describedwith reference to the second exemplary embodiment of the invention.

In fact, total image update time in respect of the embodiments of FIGS.4 and 5 can be further reduced relative to the embodiments describedwith reference to FIGS. 2 b and 3.

Referring to FIG. 6 of the drawings, a driving waveform generated by afifth exemplary embodiment of the present invention is similar in manyrespects to that described with reference to, and illustratedschematically by FIG. 5. However, in this case, a fourth sequence ofshaking pulses is generated during the time space between the firstsequence of shaking pulses and the reset pulse. By using theseadditional shaking pulses, the effects of dwell time and/or imagehistory may be further reduced, and the resulting image is of increasedquality with further reduced image retention, compared with prior artmethods. The fourth sequence of shaking pulses may have a differentformat to that of the first, second and third sequences of shakingpulses. As a result of this embodiment, the image retention can befurther reduced.

In accordance with a second aspect of the present invention, anotherdriving method is proposed. As will be apparent from the abovedescription, the inclusion of shaking pulses in the driving waveform ofan electrophoretic display is a preferred element of most, if not all,electrophoretic display driving methods (both voltage modulated andpulse width modulated). These shaking pulses increase the accuracy ofgreyscales, remove image retention, account for dwell time and, ifperformed correctly, are optically invisible to the user.

Whilst image quality is obviously a priority, there is also a need tominimise image update time, especially when changing from one greyscaleimage to another. Currently, image update times of 600-800 msec areachievable, depending on the precise details of the driving schemeemployed. However, in all driving schemes, a significant proportion ofthe image update time is taken up by shaking, as shown, for example, inFIG. 7 of the drawings, in which a sequence of shaking pulses areapplied during the image update sequence immediately prior to eachgreyscale driving pulse required to effect each greyscale transition.The shaking pulses in the illustrated waveform are an integral part ofthe image update sequences and should, ideally, be as long as possible,say at least 80 msec long and, more typically, around 160 msec, in orderto achieve the best possible image quality. Thus, shaking creates asignificant delay in the total image update time. In other words, inknown systems, there is a trade off between image quality and imageupdate times, because in order to reduce image update time shaking timemust be reduced, which has an adverse effect on image quality.

Thus, in accordance with the second aspect of the invention, it isproposed to generate shaking pulses during the dwell times between eachimage update sequence at intervals along the driving waveform,regardless of the image update signals. In this manner, image qualitycan be significantly improved and/or image update time can be reduced.As explained above, the shaking can be made optically invisible to theuser using, for example, short pulses, column inversion schemes, etc.When relatively short shaking pulses are used, data-independent shakingcan be applied to the whole display without visible optical flicker.

In a first exemplary embodiment of the second aspect of the presentinvention, a set of shaking pulses are applied at regular intervalsalong the driving waveform, during the dwell times between image updatesequences, regardless of the image update data signals, whilst the“driving” shaking pulses applied prior to the greyscale driving pulse,i.e. those which form part of the image update sequence as shown in FIG.7, remain. This is schematically illustrated in FIG. 8 forrepresentative driving waveforms for the four random greyscaletransitions as shown in FIG. 7. It is also schematically demonstrated inFIG. 8, that the dwell times t_(n), t_(n+1), t_(n+2) after differentgreyscale transitions may be different from each other.

The additional, regular shaking pulses have the effect of reducing theinfluence of these dwell times, as well as increasing greyscale accuracy(i.e. image quality). The addition of these regular shaking pulsesfurther improves image quality as the image retention is further reducedwithout increasing the total image update time, relative to the drivingmethod described with reference to FIG. 7. In other words, the adverseeffects caused by dwell time are reduced, and an increased grey levelaccuracy and reduced image retention are achieved.

These regular shaking pulses may be randomly positioned/timed withrespect to the image update sequences, although a constant time periodis preferred between two adjacent shaking pulse sequences, as denoted byt_(regular shake) in FIG. 8. Thus, the resultant shaking pulse sequencescan occur before or after an image update sequence, and they may even,sometimes, fall within an image update sequence.

The greyscale accuracy is not sensitive to the timing of these regularshaking pulses because these pulses are generally symmetric andintroduce essentially little, if any, optical disturbance, for example,if short pulses are used. In order to reduce the probability of theregular shaking having an adverse influence on greyscale accuracy, theregular shaking can be disabled while an image is being updated, andthen enabled again after the image update has been completed.

In an alternative embodiment of the second aspect of the presentinvention, the additional set of regular shaking pulses may be appliedto the display, regardless of the image update data signals, as in theembodiment described with reference to FIG. 8, whilst the “driving”shaking pulses applied prior to each greyscale driving pulse in thewaveforms illustrated in FIGS. 7 and 8, are omitted, as illustratedschematically in FIG. 9 for representative driving waveforms for thefour random greyscale transitions as shown in FIGS. 7 and 8.

Once again, the addition of the regular shaking pulses improves theimage quality as the image retention can be reduced, (almost) withoutincreasing the total image update time. Similarly, these regular shakingpulses may be randomly positioned/timed with respect to the image updatesequences, although a constant time period is preferred between twoadjacent shaking pulse sequences, as denoted by t_(regular shake) inFIG. 8. Thus, the resultant shaking pulse sequences can occur before orafter an image update sequence, and they may even, sometimes, fallwithin an image update sequence.

The omission of the “driving” shaking pulses results in a shorter totalimage update time but the dwell effects may not be completely eliminatedas the timing of the regular shaking pulses is generally not linked tothe image update sequences. This can be overcome by usingelectrophoretic material with less of a dwell time dependence.

In one exemplary embodiment of the invention, the timing of the regularshaking pulses may be such that a large number of regular shaking pulsesare applied along the driving waveforms, thereby further improving theimage quality.

Thus, in summary, the application of regular shaking pulses to drivingwaveforms for electrophoretic displays, according to the second aspectof the invention, can significantly improve image quality and/or shortenimage update time, although power consumption may be increased relativeto prior art schemes. In order to overcome this problem, and reducepower consumption, any known charge recycling technique could beapplied, particularly in respect of the regular shaking pulse functionso as to reduce the power used to charge and discharge pixel electrodesduring the shaking pulse cycling. Another option would be to providemultiple usage modes on the display device, for example, using adedicated switch enabling the device to be switched between with andwithout regular shaking. For example, the regular shaking mode may beenabled when the device is connected to a network power supply, anddisabled when the device is being used as a potable device and is,therefore, relying on its own internal power supply.

Note that the invention may be implemented in passive matrix as well asactive matrix electrophoretic displays. Also, the invention isapplicable to both single and multiple window displays, where, forexample, a typewriter mode exists. This invention is also applicable tocolour bi-stable displays. Also, the electrode structure is not limited.For example, a top/bottom electrode structure, honeycomb structure orother combined in-plane-switching and vertical switching may be used.

Embodiments of the present invention have been described above by way ofexample only, and it will be apparent to a person skilled in the artthat modifications and variations can be made to the describedembodiments without departing from the scope of the invention as definedby the appended claims. Further, in the claims, any reference signsplaced between parentheses shall not be construed as limiting the claim.The term “comprising” does not exclude the presence of elements or stepsother than those listed in a claim. The terms “a” or “an” does notexclude a plurality. The invention can be implemented by means ofhardware comprising several distinct elements, and by means of asuitably programmed computer. In a device claim enumerating severalmeans, several of these means can be embodied by one and the same itemof hardware. The mere fact that measures are recited in mutuallydifferent independent claims does not indicate that a combination ofthese measures cannot be used to advantage.

1. Display apparatus (1), comprising: an electrophoretic material (5)comprising charged particles (6) in a fluid; a plurality of pictureelements (2); first and second electrodes (3, 4) associated with eachpicture element (2) for receiving a potential difference, said chargedparticles (6) being able to occupy a position being one of a pluralityof positions between said electrodes (3, 4); and drive means arranged tosupply a sequence of picture potential differences in the form of adriving waveform for enabling said charged particles (6) to occupy oneof said positions for displaying an image, the driving waveformconsisting of a sequence of image update signals including a picturepotential difference, the image update signals being separated by dwelltimes, wherein one or more shaking pulses are generated during the dwelltimes.
 2. Display apparatus (1) according to claim 1, wherein said oneor more shaking pulses are generated following each image update signal.3. Display apparatus (1) according to claim 2, wherein said one or moreshaking pulses are generated substantially immediately following eachimage update signal.
 4. Display apparatus (1) according to claim 2,wherein each image update signal comprises a reset pulse and a greyscaledriving pulse.
 5. Display apparatus (1) according to claim 4, whereineach image update signal includes one or more shaking pulses.
 6. Displayapparatus (1) according to claim 5, wherein one or more shaking pulsesare provided prior to the reset pulse of each image update signal. 7.Display apparatus (1) according to claim 6, wherein one or more shakingpulses are provided between the reset pulse and the greyscale drivingpulse of each image update signal.
 8. Display apparatus (1) according toclaim 2, wherein a sequence of shaking pulses is generated followingeach image update signal, the energy of the shaking pulses of eachsequence decreasing progressively during said sequence.
 9. Displayapparatus (1) according to claim 1, wherein said one or more shakingpulses comprise regular shaking pulses which are generated atpredetermined intervals along said driving waveform.
 10. Displayapparatus (1) according to claim 9, wherein said intervals aresubstantially equi-distant.
 11. Display apparatus (1) according to claim9, further including charge recycling means within a power supply usedto generate said regular shaking pulses.
 12. Display apparatus (1)according to claim 9, comprising means for temporarily preventing saidregular shaking pulses from being generated during an image updatesequence, and recommencing generation of said regular shaking pulsesafter the image update sequence has been completed.
 13. Displayapparatus (1) according to claim 9, arranged and configured to operatein one of at least two modes, and further including means for switchingbetween said two modes.
 14. Display apparatus (1) according to claim 13,arranged and configured to operate in one of a first mode, in whichgeneration of said regular shaking pulses is enabled, and a second mode,in which generation of said regular shaking pulses is disabled.
 15. Amethod of driving a display apparatus (1), the apparatus comprising: anelectrophoretic material (5) comprising charged particles (6) in afluid; a plurality of picture elements (2); first and second electrodes(3, 4) associated with each picture element (2) for receiving apotential difference, said charged particles (6) being able to occupy aposition being one of a plurality of positions between said electrodes(3, 4); and drive means arranged to supply a sequence of picturepotential differences in the form of a driving waveform for enablingsaid charged particles (6) to occupy one of said positions fordisplaying an image, the driving waveform consisting of a sequence ofimage update signals including a picture potential difference, the imageupdate signals being separated by dwell times; the method including thestep of generating one or more shaking pulses during the dwell times.16. Driving apparatus for driving a display apparatus (1), the displayapparatus comprising: an electrophoretic (5) material comprising chargedparticles (2) in a fluid; a plurality of picture elements (2); first andsecond electrodes (3, 4) associated with each picture element (2) forreceiving a potential difference, said charged particles being able tooccupy a position being one of a plurality of positions between saidelectrodes (3, 4); and wherein the driving apparatus is arranged tosupply a sequence of picture potential differences in the form of adriving waveform for enabling said charged particles (6) to occupy oneof said positions for displaying an image, the driving waveformconsisting of a sequence of image update signals including a picturepotential difference, the image update signals being separated by dwelltimes, the driving apparatus further comprising means for generating oneor more shaking pulses during the dwell times.